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We used the Curie–Weiss mean-field approximation to study the effects of random single-ion anisotropy and random magnetic field in the phase diagram as well as in the thermodynamic properties of the spin-7/2 Blume–Capel model. The phase diagrams are presented in the planes temperature versus single-ion anisotropy and temperature versus magnetic field, temperature versus random parameters. The dependencies of magnetization are studied as a function of temperature and single-ion anisotropy. • Phase diagrams of the system with a random anisotropies and random field were obtained. • To investigate the system, Curie–Weiss mean-field approximation were used. • First- and second-order phase transition lines for different anisotropies were found. • Tricritical behavior for some values of anisotropy and magnetic field were observed. [ABSTRACT FROM AUTHOR]

In this paper we have studied the stabilization of the long-range order in ( z ; x ) -plane of two isotropic Heisenberg ferromagnetic monolayers coupled by a short-range exchange interaction ( J ⊥ ), by a long range dipole-dipole interactions and a magnetic field. We have applied a magnetic field along of the z -direction to study the thermodynamic properties above the critical temperature. The dispersion relation ω and the magnetization are given as function of dipolar anisotropy parameter defined as E d = ( g μ ) 2 S / a 3 J ∥ and for other Hamiltonian parameters, and they are calculated by the double-time Zubarev-Tyablikov Green's functions in the random-phase approximation (RPA). The results show that the system is unstable for values of E d ≥ 0.012 with external magnetic field ranging between H / J ∥ = 0 and 1 0 − 3 . The instability appears for E d larger then E d c = 0.0158 with H / J ∥ = 1 0 − 5 , E d c = 0.02885 with H / J ∥ = 1 0 − 4 , and E d c = 0.115 with H / J ∥ = 1 0 − 3 , i.e., a small magnetic field is sufficient to maintain the magnetic order in a greater range of the dipolar interaction. [ABSTRACT FROM AUTHOR]

We have studied the effects of the random single-ion anisotropy and random magnetic field in the phase diagram and in the thermodynamic properties of the spin-3/2 Blume–Capel model via Curie–Weiss mean-field approximation. The phase diagrams were built in the planes temperature versus single-ion anisotropy, temperature versus magnetic field, temperature versus random parameters and the dependencies of magnetization were plotted versus temperature and single-ion anisotropy. These diagrams show that, in the space ( D / J − T / J ) , the type (first- or second-order) of the phase transition between the ferromagnetic and paramagnetic phases is dependent on the random parameters. Therefore, within these conditions the model presents tricritical behavior. For large values, and a certain critical value of the random parameters, the phase transition is only of second-order, but it is of first-order within the ordered phase, between the phase with m = 1 / 2 and m = 3 / 2 , which ends in a terminal critical point. [ABSTRACT FROM AUTHOR]